Magnetic media such as used in hard disk drives and magnetic tapes allow information to be stored for long periods of time. For traditional magnetic media, data access time is limited by mechanical systems. Dynamic Random Access Memory (DRAM) and Static Random Access Memory (SRAM) have data access times on the ns scale for both read and write operations. These memories types are volatile: data is stored only as long as power is supplied to refresh the capacitor charge in DRAM and to keep the transistors on in SRAM. The need for non-volatile memory is reflected by the increasing demand for Flash memory. Flash technology has comparatively slow access times (in the μs range) and can be rewritten only a limited number of times. For these reasons, there has been keen interest in commercializing magnetoresistive random access memory (MRAM), which provides non-volatile storage and read and write speeds similar to DRAM and SRAM.
An MRAM cell is formed by a magnetic tunneling junction (MTJ), which is a structure in which two ferromagnetic layers are separated by a thin insulating barrier. When a potential difference is applied across the two ferromagnetic layers, current flows through the insulating barrier by quantum mechanical tunneling. The resistance of the MTJ depends on the relative orientation of magnetic elements in the two ferromagnetic layers. The resistance is lowest when the magnetizations are aligned in parallel and highest when they are anti-parallel. One of the relative orientations can be used to represent a “1” and the other to represent a “0”. In general, the magnetic orientation of one of the layers (the pinned layer) is kept fixed while the magnetic orientation of the other layer (the free layer) is set in a write operation. The state of the MRAM cell can be queried by measuring the junction's resistance. For an array of MRAM cells to provide reliable data storage, a sufficiently great difference in resistance between the two possible states must be realized for each cell in the array.